Wear resistant sintered alloy

ABSTRACT

An iron base sintered alloy having high wear resistance produced in a feasible and effective way, provided by compounding molybdenum disulfide or the like metal sulfide having its melting point higher than the sintering temperature of the alloy. Sulphur in the sulfide forms iron sulfide which improves wear resisting property of the alloy, while, metallic component of the sulfide diffuses throughout the base metal and serves to enhance the strength of matrix. Test results with apex seals of rotary piston engine revealed that wear amount of seals made of the alloy according to the present invention is extremely low as compared with those made of molybdenum-copper-cast iron, graphite or sintered alloy which is sintered and thereafter sulphurized by gas sulphurizing process.

United States Patent 1 Inoue 1 WEAR RESISTANT SINTERED ALLOY [75]Inventor: Hiroshi Inoue, Kashiwazaki, Japan [73] Assignee: Riken PistonRing Industrial ($0.,

Ltd., Tokyo, Japan [22] Filed: July 25, 1973 [21] Appl. No.: 382,307

[30] Foreign Application Priority Data Aug. 16. 1972 Japan 47-81410 [52]US. Cl. 29/1825; 75/203; 75/204; 75/201; 75/128 D; 75/128 P; 418/178[51] Int. Cl. ..B22F1/00 [58] Field of Search 75/203. 204, 201- 128 D.75/128 P; 29/1825; 418/178 [56] References Cited UNITED STATES PATENTS2.557.862 6/1951 Clarke. Jr. 75/118 3.177.564 4/1965 Reynolds et a129/1815 3.350.178 10/1967 Miller 29/1815 3.692.515 9/1972 Fletcher et a]75/128 3.705.020 12/1972 Nachtman 29/1815 51 Nov. 11, 1975 PrimaryExaminer-Benjamin R. Padgett Assistant E.\'zmzinerB. H. Hunt [57]ABSTRACT An iron base sintered alloy having high wear resistanceproduced in a feasible and effective way. provided by compoundingmolybdenum disulfide or the like metal sulfide having its melting pointhigher than the sintering temperature of the alloy.

Sulphur in the sulfide forms iron sulfide which improves wear resistingproperty of the alloy. while. metallic component of the sulfide diffusesthroughout the base metal and serves to enhance the strength of matrix.

Test results with apex seals of rotary piston engine revealed that wearamount of seals made of the alloy according to the present invention isextremely low as compared with those made of molybdenumcopper-cast iron.graphite or sintered alloy which is sintered and thereafter sulphurizedby gas sulphurizing process.

6 Claims, 8 Drawing Figures US. Patent Nov. 11, 1975 Sheet 1 of23,918,923

FIGJA FIGJB FIG.2A FIG.2B

FIG.2C FIG.2D

US. Patent Nov. 11, 1975 Sheet2 012 3,918,923

Fig.3

Tensile Strength (kglmm N MOS2(/o) Fig.4

Amount of Weur(micron/ hr.) m 4 NO.\ N02 N03 N04 WEAR RESISTANT SINTEREDALLOY BACKGROUND OF THE INVENTION The present invention relates to wearresistant sintered alloy suitable for high speed sliding members or thelike.

For the members sliding at high speed and in high ambient temperature,like apex seals of rotary piston engine, which are mounted on the rotarypiston and forced to slide against trochoidal inner surface of thecylinder under a circumstance where the breaking of lubricating oil filmon that sliding surface is apt to occur, less hardness decrease duringoperation and excellent oil retaining property to hold lubricating oilfilm on said sliding surface are required as well as high wearresistance.

Conventionally, cast iron of pearlitic structure has been used widely asa material of such sliding members. It is considered that theself-lubricating and oil retaining properties of graphite itselfcontained in the cast iron, together with pearlitic structure of thematrix, make the material wear resistant. Therefore, wear resistance ofsuch cast iron will be increased by increasing graphite content, butthere is a limit for the carbon content of cast iron, and it isdifficult to make the content of free graphite more than 3%.

Further, the use of graphite itself for such sliding members has beentried also, but this is unfavorable because of its rapid wear.

As is widely known, sintered alloy obtained by the powder metallurgicalprocess is porous and has good oil retaining or holding property.Therefore, excellent wear resistant material would be obtained accordingto the powder metallurgical process by adding some of the elements whichincrease wear resisting property of the material.

As a method to improve the wear resisting property of iron base alloys,the gas sulphurizing process, in which sulphur is diffused andimpregnated in the surface layer of iron or steel and thereby forms ironsulfide, has been known. However, it is difficult to apply this processto the sintered alloy at present because of such problems as corrosionof sintering furnace and generation of harmful gas, and expected resultscould not been obtained by this process under our experiments.

SUMMARY OF THE INVENTION Accordingly, an object of the present inventionis to provide a sintered alloy having high wear resistance produced in amore feasible and effective process.

Another object of the present invention is to provide a sintered alloysuitable for apex seals of rotary piston engine.

It has been found that the foregoing and related objects may be readilyattained in a sintered alloy made by a process comprising the steps ofpreparing powder mixture by weight of 1.0-1.8% carbon, (LS-2.0%chromium, 0.5-l.0% nickel, 2.0-8.0% metal sulfide or sulfides and restof iron, forming said powder mixture by compressing, and sintering saidformed powder mixture, wherein said metal sulfide or sulfides havingmelting point higher than sintering temperature of said formed powdermixture.

It has been found that sulphur in said metal sulfide or sulfidescombines with iron and forms iron sulfide while sintering which improveswear resisting property of the alloy.

Metallic component of the sulfide diffuses into the base metal andserves to enhance the strength of matrix.

It has also been found that the alloy is particularly suitable for theapex seal of rotary piston engine, and capable of forming to its shapeprecisely by machining.

Further objects, features and advantages of the present invention willbe apparent by the following description with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. HA) and KB) are micrographs (X400 X 1.5) showing the structures of one example of the sintered alloysaccording to the present invention, in which FIG. 1(A) is as polishedand FIG. 1(B) is as polished and etched by 2% HNO; alcoholic solution.

FIGS. 2(A)( D) are photographs taken by the electron microprobeanalysis, in which,

FIG. 2(A) shows absorbed electron image,

FIG. 2(B) shows L -characteristic X-ray image of molybdenum,

FIGS. 2(C) and (D) show K, -characteristic X-ray images of sulphur andiron, respectively.

FIG. 3 is a graph illustrating the variation of tensile strength of thealloy according to the present invention in relationship with differentmolybdenum disulfide contents.

FIG. 4 is a graph illustrating the amount of wear of apex seal made ofthe alloy according to the present invention in comparison with thosemade of molybdenum-copper-cast iron, graphite and sulphurized sinteredalloy respectively, after the actual engine test.

DESCRIPTION OF THE PREFERED EMBODIMEN'IS Following kinds of powder wereprepared as the raw materials of the alloy;

iron: under 100 mesh deoxidized mill scale powder,

carbon: 98% purity natural graphite powder of [0 mi crons in averagesize,

nickel: carbonyl nickel powder,

chromium: under 100 mesh ferrochromium containing 60% chromium byweight,

sulfide: molybdenum disulfide (M08 powder of 10 microns in average size.

These powders were compounded and mixed thoroughly so as to contain 1.5%carbon, 1.8% chromium, 0.7% nickel, 4% molybdenum disulfide and rest ofiron, each by weight.

The mixture was compressed under 4 ton/cm pressure to form X 10 X 6 mmsize specimens with density of 6.6 gram/cm.

The formed specimens were then sintered at 1,120C for 30 minutes in aprepared atmosphere of RX gas of 24% CO, 31% H and 45% N in averagecomposition and 0C dew point. The RX gas here means reducing gas mixtureobtained by modifying hydrocarbon gases.

Thereafter, the specimens were cooled to 500C over 40 minutes, and thencooled in furnace to room temperature.

Density of the sintered specimens was 6.0 gram/cm.

Microstructures of the specimen are shown in FIG. 1.

FIG. 1(A) is a micrograph of specimen as polished, and FIG. 1(B) is amicrograph of the same specimen etched by 2% I-INO; alcoholic solution,wherein black portions indicate pores, grey portions indicate sulfides,white portions located at the grain boundaries indicate free cementitesand matrix is pearlite.

FIGS. 2 show photographs taken by the electron microprobe analysis,wherein FIG. 2(A) indicates absorbed electron image showing the regionincluding sulfide, FIG. 2(B) indicates L a characteristic X-ray image ofmolybdenum, FIG. 2(C) indicates K a characteristic X-ray image ofsulphur and FIG. 2(D) indicates K or characteristic X-ray image of iron.

From these photographs, it will be seen that the sulfide formed is ironsulfide and molybdenum does not remain in the sulfide, but diffusesuniformly into the matrix.

FIG. 3 is a graph indicating tensile strength of the sintered alloy madeby the process mentioned above, but changing the amount of molybdenumdisulfide additron.

The tensile strength decreases with an increase of amount of molybdenumdisulfide, but it will retains about 13 kg/mm at 8% molybdenum disulfideaddition, which strength would be enough for the practical use.

Samples obtained by the above mentioned process were machined to theshape of apex seal for rotary piston engine and mounted on a rotarypiston and assembled in the center housing having a chromium platedtrochoidal inner surface, and subjected to 300 hours actual engine test.

No. 1 in FIG. 4 shows the wear amount of the tested apex seals.

Seals made of different materials were also tested for comparison.

No. 2 and No. 3 show the wear amount of seals each made ofmolybdenum-copper-cast iron and graphite respectively, both were testedfor 100 hours.

No. 4 shows the wear amount of apex seals made by the similar process asNo. 1, except that in this case molybdenum was added in the form offerromolybdenum containing 60% molybdenum and the sintered alloy wasthereafter gas sulphurized. Running test in this case was performed for300 hours.

Test results show that the apex seals according to the present inventionis extremely excellent in wear resistance as compared with those of theother three materials. Wear of the cylinder wall after the test waspractically negligible small except the case of No. 2 in which wavelikewears of about 8 micron depth were observed on the wall surface.

Referring now to each component of the alloy, carbon is a basic elementwhich imparts wear resistance and mechanical strength to the alloy. Itmakes the matrix pearlitic, further precipitates free cementite to makethe alloy endurable against hard sliding mating 3118. p Carbon contentof less than 1% will not be sufficient for such purposes, but thecontent of more than 1.8% would rather deteriorate the mechanicalproperties because of the precipitation of excessive cementite.

Chromium increases mechanical strength and wear resistance of the alloy,but it is not preferable to contain chromium more than 2%, because thealloy becomes too hard due to increase of chromium carbide content.

Nickel improves the alloy structure and increases mechanical strengthand wear resistance. However, it requires considerably high temperatureto difiuse nickel into the matrix uniformly. Therefore, it is preferableto limit nickel content to less than 1% for the present sinteringtemperature.

As for the metal sulfide, which is considered as a carrier of thesulphur into the alloy, sulfide or sulfides of such metals as aluminium,chromium, cobalt, tungsten, copper, lead and molybdenum may berecommended for the use in consideration of its melting point, affinityof each metal to sulphur comparing to that of iron, and the effect ofeach metal on the mechanical properties of the alloy.

lf melting point of a metal sulfide is lower than the sinteringtemperature, sulphur may come out of the alloy and cause not onlydecrease of sulphur content but contamination of the sinteringatmospheric gases, so that it is necessary that the melting point ofmetal sulfide is higher than the sintering temperature of usual ironbase sintered alloy which is approximately 1050 ll50C.

It is preferable to limit the quantity of metal sulfide to be added toless than 8% from the reason mainly of the adverse effect on mechanicalstrength of the alloy as shown in FIG. 3, but in case of less than 0.5%,it has little effect on wear resisting property, and it is recommendedto add more than 2% to retain remarkable effect even when mechanicalparts are used under severe conditions.

As aforementioned, the alloy of the present invention is porous and hasgood oil retaining property. Moreover, a higher proportion of ironsulfide of high wear resisting property is easily formed within the basemetal which itself is wear resistant, without any problems of such asharmful gas generation. Further, metallic component of the metal sulfidediffuses into the base metal and serves to enhance the strength ofmatrix.

Therefore, according to the present invention, the alloy of high wearresistance and good oil retaining property and still retaining enoughmechanical strength may be obtained in a feasible and effective way.

The alloy is not only suitable in use for the ordinary slidingmechanical parts, but is also particularly usable for the members usedunder severe conditions, like the apex seal of rotary piston engine, inwhich the breaking of lubricating oil film may occur due to very highspeed sliding at high ambient temperature.

Further, like molybdenum disulfide, many of the metal sulfides used inthe present invention have excellent selflubricating property, so thatthe products of high compressed density may be obtained with suchsulfides even under less compressive load and with less additionallubricant at the step of forming, and this may also enable the use ofexisting equipments. The sulfide may also improve the machinability ofsintered alloy which heretofore has been considered as inferior.

What is claimed is:

1. A method of preparing a wear-resistant iron-base alloy comprising thesteps of:

preparing a powder mixture comprising l.0l.8%

carbon, (LS-2.0% chromium, (LS-1.0% nickel, 2.0-8.0% of one or moremetal sulfides selected from a group consisting of aluminum sulfide,cobalt sulfide, tungsten sulfide, copper sulfide, lead sulfide, andmolybdenum sulfide, and the balance iron;

compressing said powder mixture into a form;

sintering said form at a temperature below the melting point of said oneor more metal sulfides;

maintaining said form at temperatures up to said sincobalt sulfide,tungsten sulfide, copper sulfide, lead tering temperature to distributethe sulfur of said sulfide, and molybdenum sulfide, and the balance oneor more metal sulfide substantially in the miiron; crosmlcmre of Saidalloy as iron sulfides, and the said alloy having a sinteredmicrostructure comprsing metal of said one or more mew] sulfidessubstan' 5 a pearlite matrix, free cementite located at grain tiallyuniformly within the matrix of said alloy; and boundaries and pores, andwherein coohng 831d form to mom mmperature the sulfur of said one ormore metal sulfide is distrib- 2. The method as claimed in claim 1,wherein said one or more metal sulfides comprises molybdenum disulfide.10

3. The method as claimed in claim 2, wherein said step of sinteringcomprises heating said form at l,lC for 30 minutes in a reducing gasatmosphere.

4 A wear resistant simered alloy prepared f a said one or more metalsulfides comprises molybdenum mixture of powders comprising: 1disulfide- 1.0-1.s% carbon, 0.s-2.0% chromium, (LS-1.0% An p seal fora yPiston engine made from nickel, 2.0-8.0% of one or more metal sulfidessethe wear resistant sintered alloy as claimed in claim 4. lected from agroup consisting of aluminum sulfide,

uted substantially in said microstructure as iron sulfide inclusions,and the metal of said one or more metal sulfides is distributedsubstantially uniformly within said matrix.

5. The sintered alloy as claimed in claim 4, wherein III

1. A METHOD OF PREPARING A WEAR-RESISTANT IRON-BASE ALLOY COMPRISING THESTEPS OF: PREPARING A POWDER MIXTURE COMPRISING 1.0-1.8% CARBON,0.5-2.0% CHROMIUM, 0.5-1.0% NICKEL, 2.0-8.0% OF ONE OR MORE METALSULFIDES SELECTED FROM A GROUP CONSISTING OF ALUMINUM SULFIDE, COBALTSULFIDE, TUNGSTEN SULFIDE, COPPER SULFIDE, LEAD SULFIDE, AND MOLYBDENUMSULFIDE, AND THE BALANCE IRON, COMPRESSING SAID POWDER MIXTURE INTO AFORM, SINTERING SAID FORM AT A TEMPERATURE BELOW THE MELTING POINT OFSAID ONE OR MORE METAL SULFIDES, MAINTAINING SAID FORM AT TEMPERATURE UPTO SAID SINTERING TEMPERATURE TO DISTRIBUTE THE SULFUR OF SAID ONE ORMORE METAL SULFIDE SUBSTANTIALLY IN THE MICROSTRUCTURE OF SAID ALLOY ASIRON SULFIDE, AND THE METAL OF SAID ONE OR MORE METAL SULFIDESSUBSTANTIALLY UNIFORMLY WITHIN THE MATRIX OF SAID ALLOY, AND COOLINGSAID FORM TO ROOM TEMPERATURE.
 2. The method as claimed in claim 1,wherein said one or more metal sulfides comprises molybdenum disulfide.3. The method as claimed in claim 2, wherein said step of sinteringcomprises heating said form at 1,120*C for 30 minutes in a reducing gasatmosphere.
 4. A WEAR RESISTANT SINTERED ALLOY PREPARED FROM A MIXTUREOF POWDERS COMPRISING: 1.0-1.8% CARBON, 0.5-2.0% CHROMIUM, 0.5-1.0%NICKEL, 2.0-8.0% OF ONE OR MORE METAL SULFIDES SELECTED FROM A GROUPCONSISTING OF ALMINUM SULFIDE, COBALT SULFIDE, TUNGSTEN SULFIDE, COPPERSULFIDE, LEAD SULFIDE, AND MOLYBDENUM SULFIDE, AND THE BLANCE IRON, SAIDALLOY HAVING A SINTERED MICROSTRUCTURE COMPRISING A PEARLITE MATRIX,FREE CEMENTITE LOCATED AT GRAIN BOUNDARIES, AND PORES, AND WHEREIN THESULFUR OF SAID ONE OR MORE METAL SULFIDE IS DISTRIBUTED SUBSTANTIALLY INSAID MICROSTRUCTURE AS IRON SULFIDE INCLUSIONS, AND THE METAL OF SAIDONE OR MORE METAL SULFIDES IS DISTRIBUTED SUBSTANTIALLY UNIFORMLY WITHINSAID MATRIX.
 5. The sintered alloy as claimed in claim 4, wherein saidone or more metal sulfides comprises molybdenum disulfide.
 6. An apexseal for a rotary piston engiNe made from the wear resistant sinteredalloy as claimed in claim 4.